Production of silica from olivine is generally covered by the co-pending Norwegian Patent Application No. 20040167.
Olivine is a natural magnesium iron silicate available in large quantities at many locations in the world. Typically it contains about 50% magnesia, about 40% silica and about 7-9% iron oxide. The olivine can contain up to 10% of accessory minerals (e.g., pyroxene, spinel, chromite and chlorite). Olivine is easily soluble in acid and it has for a long time been considered as a raw material for magnesium chemicals and silica.
U.S. Pat. No. 5,780,005 (Olerud) describes a process for production of silica from olivine, but does not mention utilisation of the remaining part of the raw material. There are several patents (e.g. WO02/48036 A1) related to the silica production ignoring utilization of remaining compounds in the olivine raw material. In order to have an economic viable process, all compounds in olivine should be utilised.
Icelandic Patent Application no. 6635 discloses production of precipitated silica with controlled specific surface area and high degree of purity. The disclosed process comprises mixing in a controlled manner olivine and a hot mineral acid solution and thereafter separating most of the undissolved olivine and accessory compounds. Resulting silica slurry is then filtered to recover the metal salt solution and the silica washed to remove dissolved salt.
The acidic metal salt solution separated from the precipitated silica and undissolved compounds, contains magnesium, iron and nickel, in addition to minor quantities of other elements, as for example manganese and aluminum. If this magnesium containing solution is to be used for the production of magnesium metal, magnesium oxide and other magnesium chemicals, it has to be purified by suitable known methods. Several methods have been described in e.g.:
U.S. Pat. No. 4,944,928 (Norsk Hydro)
K. B. Bengtson, Light Metals 1999, pp 1151-1154
U.S. Pat. No. 5,091,161 (Magnola process)
In the above purification methods a precipitate containing iron and other metallic impurities is precipitated from the magnesium salt solution. The precipitate contains nickel, which can make waste disposal costly. U.S. Pat. No. 4,944,928 also discloses a process for removal of small amounts of Ni from magnesium chloride solution, which is also applied in the present invention.